In a Long Term Evolution (LTE) system, a hybrid automatic repeat request (HARQ) mechanism is used for downlink data transmission. After user equipment (UE) receives a physical downlink shared channel (PDSCH), if the physical downlink shared channel is correctly received, the UE feeds back an acknowledgment (ACK) on a physical uplink control channel (PUCCH), and if the physical downlink shared channel is incorrectly received, the UE feeds back a negative acknowledgement (NACK) on the PUCCH. The ACK and the NACK are collectively referred to as a HARQ-ACK.
The LTE system supports a carrier aggregation (CA) technology, that is, an access network device configures a plurality of carriers for one UE, to improve a data rate of the UE. As shown in FIG. 1, a base station transmits downlink data to UE 1 by using a carrier 1 and a carrier 2, and the base station transmits downlink data to UE 2 by using a carrier 1, a carrier 3, and a carrier 5.
As shown in Table 1, there are a plurality of data channel transmission modes in a current LTE system.
TABLE 1Data channel PDSCH transmission mode in an LTE systemMode 1Cell-specific reference signalSingle-antenna portMode 2(CRS) based on antennaTransmit diversityMode 3ports 0 to 3Open-loop space divisionmultiplexMode 4Closed-loop space divisionmultiplexMode 5Multi-user multiplexMode 6Single-layer closed-loopspace division multiplexMode 7Demodulation reference signalSingle-antenna port(DMRS) based on an antennaport 5Mode 8DMRS based on antenna portsDouble current7 and 8transmissionMode 9DMRS based on antenna portsOne to eight layers of7 to 14transmission
A PDSCH scheduled in a downlink subframe in the transmission modes 1, 2, 5, 6, and 7 is a single transport block, that is, each downlink subframe corresponds to one HARQ-ACK bit, and a PDSCH scheduled in a downlink subframe in the transmission modes 3, 4, 8, and 9 may be two transport blocks, that is, each downlink subframe corresponds to two HARQ-ACK bits.
Currently, in a CA mode, to-be-aggregated carriers usually have a same PDSCH transmission mode. Therefore, bit quantities of HARQ-ACKs that need to be fed back by the UE for downlink subframes on the to-be-aggregated carriers are the same. However, as technologies develop, to-be-aggregated carriers may have different PDSCH transmission modes. For example, aggregation of a maximum of five carriers is supported currently, and aggregation of 10 carriers, 20 carriers, or even 32 carriers may appear later. In this case, to-be-aggregated carriers may have different PDSCH transmission modes, and bit quantities of HARQ-ACKs that need to be fed back for downlink subframes on different carriers are different. Currently, the UE does not support such a case in which bit quantities of HARQ-ACKs that need to be fed back for downlink subframes on different to-be-aggregated carriers are different.